Share bbPress forums across a multinetwork / multisite

I have a multinetork / multisite with Buddypress installed and BP-Multinetowrk active on some of the sites, while other sites do not have the plugin active; thereby sharing the Buddypress members, groups, group forums, etc. I also need these sites to share the same bbPress file for the main site forums. How do I configure a sites bbPress install to look to the same install as another site’s bbPress?

Unfortunately looks like this is impossible in the default version.
I need this too but I have not found any decision. And seems that the developers do not make any steps in this direction, so that hardly expect it soon. Use built-in Buddypress group forums are now the only solution.
I will be very happy to know that I was wrong!)